Process for treating the surface of a stretched film

ABSTRACT

This invention relates to a process for treating the surface of a stretched film of a polyester or polypropylene or copolymers or polymer mixtures of the latter containing at least 60 percent by weight, calculated on the total polymer weight, of propylene, in order to improve the adhesion of the film surface to a heatsealable coating, which comprises subjecting the film surface to a corona discharge in an atmosphere consisting essentially of nitrogen or carbon dioxide containing not more than about 15 percent by volume of oxygen, at a film temperature in the range of room temperature to about 25* to 50* C. below the softening point of the stretched film. The invention also relates to the films so treated.

I United States Patent 1151 3,639,134 Stegmeier et al. 1 Feb. 1, 1972[54] PROCESS FOR TREATING THE [561 References Cited SURFACE OF ASTRETCHED FILM UNITED STATES PATENTS 1 lhvemow (third 8M" Maire-When;3,030,290 4/1962 Ryan ..117/138.8 x "9 8- hnlwjt, \Ylesbaden; Gebkr,3,088,844 5/1963 Hungerford et al.... .11 17/47 W w n 119m Dimer,3,309,299 3/1967 Mantel] .204/165 Frankfurt-Rocha, all of Germany3,403,042 9/l968 Stead ..117/7 Alt esel ml Ass'gnee gfi' 3:22 w'esbadenPrimary Examiner-William D. Martin Assistant Examiner-Theodore G. Davis[22] Filed: June 4, 1969 Attorney-James E. Bryan [211 App]. No.: 830,505[57] ABSTRACT This invention relates to a process for treating thesurface of a [30] Foreign Appnuuon Priority Data stretched film of apolyester or polypropylene or copolymers June 6, 1968 Germany ..P 17 69534.0 or P y mixtures of the latter containing at least 60 9 July29,1968 Germany ..1 17 69 872.5 cent y weight, calculated on the total Py weight. of

propylene, in order to improve the adhesion of the film surs2] u.s.ci...117/7,117/47 A, 117/931, face to heat-scalable coating, whichcomprises subjecting 17 3 3 15 17/13 3 p the film surface to a coronadischarge in an atmosphere consisting essentially of nitrogen or carbondioxide containing not more than about 15 percent by volume of oxygen,at a film [51] Int. Cl. ..B44d 5/04, B32b 27/32 temperature in the rangeof room temperature to about 25 to [58] Field of Search ..1 17/7, 138.8F, l38.8 E, 47 A, 50 C. below the softening point of the stretched film.The inll7/93.1 CD, 122 H; 204/165, 168; 250/495 TC vention also relatesto the films so treated.

7 Claims. No Drawings PROCESS FOR TREATING TIIE SURFACE OF A STRETCIIEDFILM This invention relates to a process for treating the surface of astretched film of polypropylene, or copolymers or polymer mixturesthereof containing at least 60 percent by weight, calculated on thetotal polymer weight, of propylene, or of polyesters, such aspolyethylene terephthalate. Especially, the film is a biaxiallystretched film of isotactic polypropylene which is treated to improvethe adhesion of a heat-scalable coating, particularly one containing oneor. more polyvinylidene chloride copolymers, by means of a coronatreatment at room temperature or at an elevated temperature in thepresence of gases.

It is known that a layer of a polyvinylidene chloride copolymer has anunsatisfactoryadhesion to an untreated surface of a stretched film ofmaterials of the aforementioned type, particularly when the layer hasbeen applied from an aqueous dispersion. Pretreatment methods have thusbeen employed, of which two types have been widely accepted, viz, theactivation of the surface by means of a flame treatment and theactivation of the surface by means of a corona treatment.

Corona treatment generally is effected in an atmosphere of air and/or ahalogen. It is also known in the case of a corona discharge in anatmosphere of air to improve the adhesion by raising the temperature.However, the improvements are insufficient for most fields ofapplication, particularly when the discharge is effected at mediumfrequencies. Improved adhesion values useful for practical applicationare achieved by the corona treatment in the presence of halogens. Inaddition to the increased reactivity, the toxicity, and the odor, thispretreatment involves numerous further disadvantages, how ever.Considerable equipment is necessary for sealing off and destroying thegas. Furthermore, particularly when working at elevated temperatures,considerable corrosion of the apparatus occurs so that the processtechnically can be performed only with difficulties.

The present invention overcomes the aforementioned disadvantages andprovides a process for improving the adhesion to a heat-sealable coatingof the surface of a stretched film of polypropylene, or copolymers orpolymer mixtures thereof, having at least 60 percent by weight,calculated on the total polymer weight, of propylene, or of polyesters,such as polyethylene terephthalate, particularly, however, a biaxiallystretched film of isotactic polypropylene.

This is achieved by performing a corona treatment of the film surface atroom temperature or at an elevated temperature in the presence of gases.In the process of the invention, the corona treatment is performed inthe presence of nitrogen or carbon dioxide, or nitrogen/air mixtures, orcarbon dioxide/air mixtures, with oxygen contents of up to about 15,preferably up to 10, percent byvolurne at film temperatures in the rangeof room temperature to about 25 to 50 C. below the softening point ofthe stretched film.

The discharge frequencies during treatment are in the range of about.0.1 to 1,000 kilocycleslsecond, preferably in the range from I to Ikilocycles/second.

Suitable coating materials are, for example, high-pressure orlow-pressure polyethylenes, polyvinylidene chloride, polyvinylchloride/polyvinyl acetate copolymers, polyvinyl chloride, and thecopolymers or polymer mixtures thereof, which are applied by meltextrusion as a film or from a solution. Especially suitable, however,are aqueous dispersions, the application of an aqueous polyvinylidenechloride copolymer dispersion being especially preferred.

Surprisingly, it has been found that a corona treatment in the presenceof nitrogen or carbon dioxide involves an improvement of the adhesion ofthe coating to the surface of the listed films. This effect also occurswhen the oxygen concentration of air is decreased in the corona space,the decrease being achieved in a simple manner by blowing pure orcommercial nitrogen or carbon dioxide into the corona space.

Considerable adhesion improvement is additionally achieved by heatingthe stretched film from room temperature, at which technically usefulresults also are achieved, to higher temperatures up to about 25 to 50C. below the softening point of the film to be treated. In accordancewith the invention, in the case of biaxially stretched films ofisotactic polypropylene, for example, treatment preferably is performedat temperatures in the range of about 75 to 1 10 C. In the case ofpolyethylene terephthalate films, temperatures in the range from about75 to 160 C. are employed.

The temperature'may beincreased in the usual way, e.g., by anappropriately'heated rollermounted in front of the corona space and/orby superposedinfrared radiation and/or by preheating the, gas blown intothe corona space.

The travelling speeds of the films to be treated are not critical forthe process of the invention and may range between about 5 and I00m./min. or more.'They can be conventionally adjusted to the conditionsnecessary in accordance with the invention, such as temperature,treatment intensity, and the coatings, with respect to the knownpretreatments in air or chlorine, will be further illustrated by thefollowing examples.

MEASUREMENT OF THE HEAT- SEALING VALUES GIVEN IN THE EXAMPLES Theadhesion is evaluated by the measurement of the heatsealing values.Sealing is performed for 2 seconds in a Sentinel apparatus at l20 C.under a pressure of 0.4 kg./cm. The measurement of the heat-sealingstrength is carried out by means of a tension tester, the draw off speedof which is 200 mmJmin.

EXAMPLE I At a feed speed of 50 m./min., a biaxially stretched 20p.thick polypropylene film was exposed for about l/50 second to a coronadischarge. Nitrogen was intensively supplied into the corona space insuch a manner that an oxygen concentration of 10 percent by volume wasobtained. The discharge frequency was 5 kilocycles/second, the distanceof the electrodes from the film was about 0.5 mm. and the voltagebetween the electrodes and the roller serving as an antipole was about6,000 volts.

At a web speed of 30 m./min., the film then was coated with an aqueouspolyvinylidene chloride dispersion and dried by means of hot air at 135C. The applied layer thickness was 1.5;!" Heat-sealing value: g./20 mm.

COMPARATIVE TESTS The procedure was the same as that described inexample 1 except that the corona discharge was effected in a chlorineatmosphere. Heat-sealing value: g./20 mm.

The procedure was the same as that described in example 1 except thatthe corona discharge took place in an atmosphere of air. Heat-sealingvalue: 60 g./20 mm.

EXAMPLE 2 The procedure of example 1 was repeated with a 20p. thickpolypropylene film with the exception that the film was so heated by ahot roller mounted in front of the corona space that it had atemperature of 80 C. at the time of the corona discharge.

Heat-sealing value: g./20 min. (only the values at rupture could bemeasured).

COMPARATIVE TEST The procedure of example 2 was performed in air.Heatsealing value: 70 g./20 mm.

EXAMPLE 3 A biaxially stretched 201;. thick polypropylene film wastreated in a laboratory test in a closed system at 80 C. for 1 second ata discharge frequency of kilocycles/second. The

treatment was performed in an atmosphere of pure nitrogen COMPARATIVETEST The procedure was the same as that described in example 3 exceptthat it was performed in air, Heat-sealing value: 40 to 50 g./2O mm.

The absolute values of example 3 cannot be compared to those of example2 since, in the case of example 3, the procedure was performedin alaboratory apparatus.

EXAMPLE 4 At a web speed of 50 m./min., a biaxially stretched thickpolypropylene film was exposed to a corona discharge. Nitrogen wasintensively passed into the corona space. The discharge frequency was 60kilocycles/second.

As described in example 1, the film was then provided with a 1.5 1.thick polyvinylidene chloride coating from an aqueous dispersion. I

Heat-sealing value: 80 g./2O mm.

COMPARATIVE TEST EXAMPLE 5 All films obtained according to examples 1 to4 and to the comparative tests were exposed for 7 days to a temperatureof C. and a relative humidity of 85 percent. No film showed a decreaseof the heat-sealing value.

EXAMPLE 6 The procedure was analogous to that of example l except thatcarbon dioxide was used instead of nitrogen.

Coating was then effected at a web speed of 30 m./min. using an aqueouspolyvinylidene chloride dispersion and drying was effected with hot airat C. The thickness of the layer applied wasl .Su. Heat-sealing value:90 g./20 mm.

EXAMPLE 7 The procedure of example 2 was repeated using a 20p. thickpolypropylene film, with the exception that carbon dioxide was passedinto the corona space instead of nitrogen.

Heat-sealing value: g./20 mm. (value at rupture).

EXAMPLE 8 The procedure of example 7 was repeated with the exceptionthat coating was efiected with an aqueous dis ersion of a copolymer ofvinyl chloride and vinyl acetate. eat-sealing value: 140 g./20 mm.(value at rupture).

COMPARATIVE TEST The procedure was the same as that of example 8 exceptthat the corona discharge was effected at 80 C. in air. Heatsealingvalue: 30 g./20mm.

Similar good results in improving the adhesion according to the processof the invention were obtained with stretched films from copolymers andpolymer mixtures containing at least 60 percent by weight, calculated onthe total polymer weight, of polypropylene, as well as with polyesterfilms, particularly with polyethylene terephthalate films.

lt will-'b'e obvious to those skilled in the art that many modificationsmay be made within the scope of the present invention without departingfrom the spirit thereof, and the invention includes all suchmodifications.

What is claimed is:

L A process for treating the surface of a stretched film of a polyesteror polypropylene or copolymers or polymer mixtures of the lattercontaining at least 60 percent by weight, calculated on the totalpolymer weight, of propylene, which comprises subjecting the filmsurface to a corona discharge in an atmosphere consisting essentially ofnitrogen or carbon dioxide containing not more than about 15 percent byvolume of oxygen, at a film temperature in the range of room temperatureto about 25 to 50 C. below the softening point of the stretched film andapplying a heat-scalable coating to the treated film.

2. A- process according to claim 1 in which the film is isotacticpolypropylene.

3. A process according to claim 1 in which the heat-scalable coatingcomprises a vinylidene chloride polymer.

4. A process according to claim 1 in which the treatment atmospherecontains not more than about 10 percent by volume of oxygen.

5. A process according toclaim 1 in which the treatment atmosphere is amixture of nitrogen and air.

6. A process according to claim 1 in which the treatment atmosphere is amixture of carbon dioxide and air.

7. A process according to claim 1 in which the corona discharge has afrequency in the range of about 0.1 to 1,000 kilocycles/second.

2. A process according to claim 1 in which the film is isotacticpolypropylene.
 3. A process according to claim 1 in which theheat-sealable coating comprises a vinylidene chloride polymer.
 4. Aprocess according to claim 1 in which the treatment atmosphere containsnot more than about 10 percent by volume of oxygen.
 5. A processaccording to claim 1 in which the treatment atmosphere is a mixture ofnitrogen and air.
 6. A process according to claim 1 in which thetreatment atmosphere is a mixture of carbon dioxide and air.
 7. Aprocess according to claim 1 in which the corona discharge has afrequency in the range of about 0.1 to 1,000 kilocycles/second.